We compared kinetic indices of pulmonary surfactant metabolism in premature infants (n ‫؍‬ 41) with respect to i ) tracer ([1-13 C 1 ]acetate, [U-13 C 6 ]glucose, and [1,2,3,4-13 C 4 ] palmitate), ii ) phospholipid (PL) pool (total PLs or disaturated PLs), or iii ) instrumentation [gas chromatography/ mass spectrometry (GC/MS) or GC-combustion-isotope ratio mass spectometry (GC-C-IRMS)]. Tracer incorporation was measured in PLs extracted from serial tracheal aspirates after a 24 h tracer infusion. The fractional catabolic rate (FCR), representing the total fractional turnover from all sources of surfactant production, was independent of tracer. The fractional synthesis rate of surfactant PL from plasma palmitate was significantly higher than that from palmitate synthesized de novo from acetate, and these two sources of palmitate together accounted for only half of the total surfactant production in preterm infants. [U-13 C 6 ]glucose showed significant recycling of the 13 C label in intermediary metabolism, distinguishable by GC-MS but not by GC-C-IRMS, resulting in a slower apparent FCR when GC-C-IRMS was used. The extracted PL pool did not affect the surfactant metabolic indices. We suggest that FCR should be used as a primary measure of surfactant turnover kinetics and that tracers labeling both de novo synthesis (acetate and glucose) and preformed pathways (plasma palmitate) can be used to partition the fractional contribution of each pathway to total production. -Bohlin, K., B.

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